In the field of silver halide photography, a technique in which positive photographic images are obtained without using a negative image intermediate, or an intermediate process producing a negative image, is called direct positive photography, and photographic light-sensitive materials and photographic emulsions using such a photographic technique are called direct positive light-sensitive photographic materials and direct positive photographic emulsions, respectively.
A variety of direct positive photographic techniques are known. The most useful methods are methods in which silver halide grains which have previously been fogged are exposed to light in the presence of a desensitizer followed by development, and methods comprising exposing a silver halide emulsion containing silver halide grains having light-sensitive specks mainly inside the silver halide grains to light and then developing the exposed emulsion in the presence of a fogging agent. The present invention relates to the latter technique. Silver halide emulsions possessing light-sensitive specks in the inside of the silver halide grains and forming latent images mainly inside the grains are referred to as internal latent image type silver halide emulsions, and thus are distinguished from silver halide grains which form latent images mainly on the surface of the grains.
Methods for obtaining direct positive images by surface-developing an internal latent image type silver halide photographic emulsion in the presence of a fogging agent, and photographic emulsions and photographic light-sensitive materials employed in such methods are disclosed in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552 and 3,761,276, British Pat. Nos. 1,011,062 and 1,151,363, Japanese Patent Publication No. 29405/68, etc.
In the internal latent image type method for obtaining direct positive images, the fogging agent can be incorporated into a developing solution. However, by incorporating the fogging agent into the photographic emulsion layers or associated layers of the photographic light-sensitive material, thereby absorbing the fogging agent onto the surface of the silver halide grains, better reversal characteristics can be obtained.
Fogging agents which can be employed in the above-described method for obtaining direct positive image include hydrazine and derivatives thereof as described in U.S. Pat. Nos. 2,563,785, 2,588,982 and 3,227,552, respectively. In particular, U.S. Pat. No. 3,227,552 discloses that hydrazide and hydrazine type compounds which are derivatives of hydrazine can be incorporated not only in the developing solution, but also in the light-sensitive layers. Also, fogging agents comprising heterocyclic quaternary salt compounds described in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738 and 3,759,901, Japanese Patent Application (OPI) Nos. 3426/77 and 69613/77 have been known.
Further, it has been proposed in U.S. Pat. No. 4,030,925 (corresponding to German Patent Application (OLS) No. 2,635,316) and U.S. Pat. No. 4,031,127 (corresponding to German Patent Application (OLS) No. 2,635,317) that acyl hydrazinophenylthiourea compounds be employed.
However, these known fogging agents are accompanied by several disadvantages, viz., they have an adverse influence on preservability of the direct positive photographic light-sensitive materials, they are deficient in fogging ability for internal latent image type silver halide grains having small particle size, their reversal characteristics vary greatly depending upon changes in bromine ion concentration in the developing solution used, and their reversal characteristics vary widely depending upon changes in the amount thereof used.